1. Field of the Invention
The present invention relates to a system and method for the recovery of germanium from Modified Chemical Vapor Deposition (MCVD) processing wastes by chemical conversion by contact with a gas and for recovery of germanium from the solid cake product.
2. Description of the Related Art
Presently, optical components such as optical fibers are being manufactured through a sequence of manufacturing steps which include the reactions of a silicon containing gas, such as silicon tetrachloride, and reactions of a germanium containing gas, such as germanium tetrachloride, to form a glass with desirable optical properties. In such processes, the effluents from the induced germanium and silicon reactions are vented to the atmosphere. Such effluents typically included a significant amount of particulate as well as gaseous materials. For example, generally in typical manufacturing processes SiCl4 reacts with O2 (as in the MCVD process described in J. B. MacChesney, xe2x80x9cMaterials and Processes for Preform Fabrication-Modified Chemical Vapor Deposition,xe2x80x9d Proceedings of IEEE, 68, 1181-1184 (1980)) or O2 and H2 (as in the soot process described in P. C. Shultz, xe2x80x9cFabrication of Optical Waveguides by the Outside Vapor Deposition Process,xe2x80x9d Proceedings of IEEE, 68, 1187-1190 (1980) or as in the VAD process described in T. Izawa and N. Inagaki, xe2x80x9cMaterials and Processes for Fiber Preform Fabrication-Vapor Phase Axial Deposition,xe2x80x9d Proceedings of IEEE, 68, 1184-1187 (1980)) to produce particulate SiO2 with close to 100 percent efficiency and with approximately 50 percent incorporation of these SiO2 particulates into the glass body being fabricated.
In the production of optical waveguide materials by the Modified Chemical Vapor Deposition (MCVD) process, a large quantity of high purity germanium tetrachloride is used. A considerable portion (about 80%) of this expensive reagent appears in the MCVD effluent stream as unreacted GeCl4 and amorphous SiO2 particulates which contain upwards of 15 wt % germanium. This inefficiency coupled with increasing costs incurred for optical grade GeCl4 led to the implementation of a process for recovering germanium from the MCVD effluent stream.
As noted above, the gaseous portion of the effluent contains a large portion of the discarded germanium. By removing the germanium in these gases and collecting it in a state suitable for recycling, a substantial reduction in the cost of manufacturing optical fibers is achieved. An effective means for removing the germanium from the gas phase and, incidentally, any germanium in particulate form, involves a recirculating process as discussed in U.S. Pat. No. 4,385,915, which is herein incorporated by reference. In this process, the manufacturing effluent is treated with an aqueous medium to ensure dissolution of germanium containing gases such as GeCl4 through the processes of absorption and hydrolysis of the absorbed gas in the medium. The aqueous medium is filtered of particulates and recycled for addition of more effluent and for refiltration. Through the use of a basic or acidic media, a substantial amount of the particulate matter is also dissolved prior to filtration. Thus, the concentration of germanium in the medium is substantially increased by the recycling process and by the dissolution of germanium containing particulates. A relatively small percentage of the recycling medium is periodically or continuously removed for precipitation of germanium. The removed portion is treated with a multivalent cation such as Mg+2 to induce the formation of a corresponding precipitate such as MgGeO3. The precipitate is then separated from the remaining liquid by conventional means.
Conventional methods for processing germanium in filter cake include conventional industrial level processing of germanium concentrates derived from Ge-bearing ores and other germanium raw materials. Well-established industrial methods for the production of GeCl4 from Ge-bearing ores invariably utilize concentrated liquid hydrochloric acid for the chemical treatment of germanium concentrates. The GeCl4 produced is then separated by azeotropic distillation from aqueous HCl. The prior methods for the recovery of germanium had several drawbacks. For example, the prior methods could not efficiently process filter cakes having high water content. The prior recovery methods also added additional liquid to the system which had to be removed by drying or other methods. The prior methods also required thermal energy to be input to separate the germanium product from the filter cake. In addition, in order to separate GeCl4 from a host of associated elements (As, Sb, Sn, etc.) that may be present in the concentrates, the distillation of GeCl4 according to the prior methods needed to be carried out under oxidizing conditions (Cl2, H2O2, KMNO4, Cu, etc.) which converted these constituents to non-volatile forms. The present invention overcomes these drawbacks.
The present invention relates to a system and method for the recovery of germanium from the MCVD processing wastes by chemical conversion from a solid cake product. In the present method, the direct reaction of gaseous hydrogen chloride with the MCVD wastes (filter cake) effects the rapid and complete chlorination of the germanates, yielding germanium tetrachloride. The germanium tetrachloride product is completely volatilized and removed from the mixture during the exothermic process. The GeCl4 product may then be recovered by cryogenic methods.
The above and other advantages and features of the invention will be more clearly understood from the following detailed description which is provided in connection with the accompanying drawings.